Loss of SLC25A46 causes neurodegeneration by affecting mitochondrial dynamics and energy production in mice

Ucp4 Knockdown of Cerebellar Purkinje Cells Induces Bradykinesia

Although uncoupling protein 4 (UCP4) is the most abundant protein reported in the brain, the biological function of UCP4 in cerebellum and pathological outcome of UCP4 deficiency in cerebellum remain obscure. To evaluate the role of Ucp4 in the cerebellar Purkinje cells (PCs), we generated the conditional knockdown of Ucp4 in PCs (Pcp2cre;Ucp4fl/fl mice) by breeding Ucp4fl/fl mice with Pcp2cre mice. Series results by Western blot, immunofluorescent staining, and triple RNAscope in situ hybridization confirmed the specific ablation of Ucp4 in PCs in Pcp2cre;Ucp4fl/fl mice, but did not affect the expression of Ucp2, the analog of Ucp4. Combined behavioral tests showed that Pcp2cre;Ucp4fl/fl mice displayed a characteristic bradykinesia in the spontaneous movements. The electromyogram recordings detection excluded the possibility of hypotonia in Pcp2cre;Ucp4fl/fl mice. And the electrical patch clamp recordings showed the altered properties of PCs in Pcp2cre;Ucp4fl/fl mice. Moreover, transmission electron microscope (TEM) results showed the increased mitochondrial circularity in PCs; ROS probe imaging showed the increased ROS generation in molecular layer; and finally, microplate reader assay showed the significant changes of mitochondrial functions, including ROS, ATP, and MMP in the isolated cerebellum tissue. The results suggested that the specific knockdown of mitochondrial protein Ucp4 could damage PCs possibly by attacking their mitochondrial function. The present study is the first to report a close relationship between UCP4 deletion with PCs impairment, and suggests the importance of UCP4 in the substantial support of mitochondrial function homeostasis in bradykinesia. UCP4 might be a therapeutic target for the cerebellar-related movement disorder.

Identification of SLC25A46 interaction interfaces with mitochondrial membrane fusogens Mfn2 and Opa1

Mitochondrial fusion requires the sequential merger of four bilayers to two. The outer-membrane solute carrier protein SLC25A46 interacts with both the outer and inner-membrane dynamin family GTPases Mfn1/2 and Opa1. While SLC25A46 levels are known affect mitochondrial morphology, how SLC25A46 interacts with Mfn1/2 and Opa1 to regulate membrane fusion is not understood. In this study, we use crosslinking mass-spectrometry and AlphaFold 2 modeling to identify interfaces mediating a SLC25A46-Opa1-Mfn1/2 complex. We reveal that the bundle signaling element of Opa1 interacts with SLC25A46, and the helical repeat 1 region of Mfn2 interacts with the SLC25A46 N-terminus. We validate these newly identified interaction interfaces and show that they play a role in mitochondrial network maintenance.

Anesthetic Management for a Child With a Newly Identified Mitochondrial Disease SLC25A46 Mutation: A Case Report

SLC25A46 mutation is a newly recognized mitochondrial mutation causing neurological and muscular abnormalities. We describe a first-ever report of the anesthetic management of a seven-year-old boy with an SLC25A46 mutation during a major orthopedic procedure. The patient was nonverbal and presented with cerebral visual impairment, torticollis, and lower extremity contractures. Because of his new diagnosis of mitochondrial disease and history of delayed awakening after anesthesia, we performed general anesthesia with sevoflurane, a low-dose ketamine infusion, and small doses of fentanyl while avoiding propofol and maintaining normoglycemia and normothermia. No postoperative complications were noted during the recovery period.

Application of super-resolution microscopy in mitochondria-dynamic diseases

Limited by spatial and temporal resolution, traditional optical microscopy cannot image the delicate ultra-structure organelles and sub-organelles. The emergence of super-resolution microscopy makes it possible. In this review, we focus on mitochondria. We summarize the process of mitochondrial dynamics, the primary proteins that regulate mitochondrial morphology, the diseases related to mitochondrial dynamics. The purpose is to apply super-resolution microscopy developed during recent years to the mitochondrial research. By providing the right research tools, we will help to promote the application of this technique to the in-depth elucidation of the pathogenesis of diseases related to mitochondrial dynamics, assistdiagnosis and develop the therapeutic treatment.

The complexities of investigating mitochondria dynamics in multiple sclerosis and mouse models of MS

Multiple sclerosis (MS) is a demyelinating, degenerating disorder of the central nervous system (CNS) that is accompanied by mitochondria energy production failure. A loss of myelin paired with a deficit in energy production can contribute to further neurodegeneration and disability in patients in MS. Mitochondria are essential organelles that produce adenosine triphosphate (ATP) via oxidative phosphorylation in all cells in the CNS, including neurons, oligodendrocytes, astrocytes, and immune cells. In the context of demyelinating diseases, mitochondria have been shown to alter their morphology and undergo an initial increase in metabolic demand. This is followed by mitochondrial respiratory chain deficiency and abnormalities in mitochondrial transport that contribute to progressive neurodegeneration and irreversible disability. The current methodologies to study mitochondria are limiting and are capable of providing only a partial snapshot of the true mitochondria activity at a particular timepoint during disease. Mitochondrial functional studies are mostly performed in cell culture or whole brain tissue, which prevents understanding of mitochondrial pathology in distinct cell types in vivo. A true understanding of cell-specific mitochondrial pathophysiology of MS in mouse models is required. Cell-specific mitochondria morphology, mitochondria motility, and ATP production studies in animal models of MS will help us understand the role of mitochondria in the normal and diseased CNS. In this review, we present currently used methods to investigate mitochondria function in MS mouse models and discuss the current advantages and caveats with using each technique. In addition, we present recently developed mitochondria transgenic mouse lines expressing Cre under the control of CNS specific promoters to relate mitochondria to disease in vivo.

2D MoS2 and BN Nanosheets Damage Mitochondria through Membrane Penetration

With the progression of nanotechnology, a growing number of nanomaterials have been created and incorporated into organisms and ecosystems, which raises significant concern about potential hazards of these materials on human health, wildlife, and the environment. Two-dimensional (2D) nanomaterials are one type of nanomaterials with thicknesses ranging from that of a single atom or of several atoms and have been proposed for a variety of biomedical applications such as drug delivery and gene therapy, but the toxicity thereof on subcellular organelles remains to be studied. In this work, we studied the impact of two typical 2D nanomaterials, MoS₂ and BN nanosheets, on mitochondria, which are a type of membranous subcellular organelle that provides energy to cells. While 2D nanomaterials at a low dose exhibited a negligible cell mortality rate, significant mitochondrial fragmentation and partially reduced mitochondrial functions occurred; cells initiate mitophagy in response to mitochondrial damages, which cleans damaged mitochondria to avoid damage accumulation. Moreover, the molecular dynamics simulation results revealed that both MoS₂ and BN nanosheets can spontaneously penetrate the mitochondrial lipid membrane through the hydrophobic interaction. The membrane penetration induced heterogeneous lipid packing resulting in damages. Our results demonstrate that even at a low dose 2D nanomaterials can physically damage mitochondria by penetrating the membrane, which draws attention to carefully evaluating the cytotoxicity of 2D nanomaterials for the potential biomedical application.

Decoding Diabetes Biomarkers and Related Molecular Mechanisms by Using Machine Learning, Text Mining, and Gene Expression Analysis

The molecular basis of diabetes mellitus is yet to be fully elucidated. We aimed to identify the most frequently reported and differential expressed genes (DEGs) in diabetes by using bioinformatics approaches. Text mining was used to screen 40,225 article abstracts from diabetes literature. These studies highlighted 5939 diabetes-related genes spread across 22 human chromosomes, with 112 genes mentioned in more than 50 studies. Among these genes, HNF4A, PPARA, VEGFA, TCF7L2, HLA-DRB1, PPARG, NOS3, KCNJ11, PRKAA2, and HNF1A were mentioned in more than 200 articles. These genes are correlated with the regulation of glycogen and polysaccharide, adipogenesis, AGE/RAGE, and macrophage differentiation. Three datasets (44 patients and 57 controls) were subjected to gene expression analysis. The analysis revealed 135 significant DEGs, of which CEACAM6, ENPP4, HDAC5, HPCAL1, PARVG, STYXL1, VPS28, ZBTB33, ZFP37 and CCDC58 were the top 10 DEGs. These genes were enriched in aerobic respiration, T-cell antigen receptor pathway, tricarboxylic acid metabolic process, vitamin D receptor pathway, toll-like receptor signaling, and endoplasmic reticulum (ER) unfolded protein response. The results of text mining and gene expression analyses used as attribute values for machine learning (ML) analysis. The decision tree, extra-tree regressor and random forest algorithms were used in ML analysis to identify unique markers that could be used as diabetes diagnosis tools. These algorithms produced prediction models with accuracy ranges from 0.6364 to 0.88 and overall confidence interval (CI) of 95%. There were 39 biomarkers that could distinguish diabetic and non-diabetic patients, 12 of which were repeated multiple times. The majority of these genes are associated with stress response, signalling regulation, locomotion, cell motility, growth, and muscle adaptation. Machine learning algorithms highlighted the use of the HLA-DQB1 gene as a biomarker for diabetes early detection. Our data mining and gene expression analysis have provided useful information about potential biomarkers in diabetes.

Mitochondrial protein dysfunction in pathogenesis of neurological diseases

Mitochondria are essential organelles for neuronal function and cell survival. Besides the well-known bioenergetics, additional mitochondrial roles in calcium signaling, lipid biogenesis, regulation of reactive oxygen species, and apoptosis are pivotal in diverse cellular processes. The mitochondrial proteome encompasses about 1,500 proteins encoded by both the nuclear DNA and the maternally inherited mitochondrial DNA. Mutations in the nuclear or mitochondrial genome, or combinations of both, can result in mitochondrial protein deficiencies and mitochondrial malfunction. Therefore, mitochondrial quality control by proteins involved in various surveillance mechanisms is critical for neuronal integrity and viability. Abnormal proteins involved in mitochondrial bioenergetics, dynamics, mitophagy, import machinery, ion channels, and mitochondrial DNA maintenance have been linked to the pathogenesis of a number of neurological diseases. The goal of this review is to give an overview of these pathways and to summarize the interconnections between mitochondrial protein dysfunction and neurological diseases.

Light-activated mitochondrial fission through optogenetic control of mitochondria-lysosome contacts

Mitochondria are highly dynamic organelles whose fragmentation by fission is critical to their functional integrity and cellular homeostasis. Here, we develop a method via optogenetic control of mitochondria–lysosome contacts (MLCs) to induce mitochondrial fission with spatiotemporal accuracy. MLCs can be achieved by blue-light-induced association of mitochondria and lysosomes through various photoactivatable dimerizers. Real-time optogenetic induction of mitochondrial fission is tracked in living cells to measure the fission rate. The optogenetic method partially restores the mitochondrial functions of SLC25A46^−/− cells, which display defects in mitochondrial fission and hyperfused mitochondria. The optogenetic MLCs system thus provides a platform for studying mitochondrial fission and treating mitochondrial diseases.

Existing methods can lack spatiotemporal accuracy to manipulate dynamic mitochondrial behaviour in live cells. Here the authors report an optogenetic method to control mitochondria-lysosome contacts and induce mitochondrial fission; they use photoactivatable dimerizers including CRY2/CIB and SspB/iLID.

Genome-wide analysis identified novel susceptible genes of restless legs syndrome in migraineurs

Background

Restless legs syndrome is a highly prevalent comorbidity of migraine; however, its genetic contributions remain unclear.

Objectives

To identify the genetic variants of restless legs syndrome in migraineurs and to investigate their potential pathogenic roles.

Methods

We conducted a two-stage genome-wide association study (GWAS) to identify susceptible genes for restless legs syndrome in 1,647 patients with migraine, including 264 with and 1,383 without restless legs syndrome, and also validated the association of lead variants in normal controls unaffected with restless legs syndrome (n = 1,053). We used morpholino translational knockdown (morphants), CRISPR/dCas9 transcriptional knockdown, transient CRISPR/Cas9 knockout (crispants) and gene rescue in one-cell stage embryos of zebrafish to study the function of the identified genes.

Results

We identified two novel susceptibility loci rs6021854 (in VSTM2L) and rs79823654 (in CCDC141) to be associated with restless legs syndrome in migraineurs, which remained significant when compared to normal controls. Two different morpholinos targeting vstm2l and ccdc141 in zebrafish demonstrated behavioural and cytochemical phenotypes relevant to restless legs syndrome, including hyperkinetic movements of pectoral fins and decreased number in dopaminergic amacrine cells. These phenotypes could be partially reversed with gene rescue, suggesting the specificity of translational knockdown. Transcriptional CRISPR/dCas9 knockdown and transient CRISPR/Cas9 knockout of vstm2l and ccdc141 replicated the findings observed in translationally knocked-down morphants.

Conclusions

Our GWAS and functional analysis suggest VSTM2L and CCDC141 are highly relevant to the pathogenesis of restless legs syndrome in migraineurs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01409-9.

Proteomic Identification of the SLC25A46 Interactome in Transgenic Mice Expressing SLC25A46-FLAG

The outer mitochondrial membrane protein SLC25A46 has been recently identified as a novel genetic cause of a wide spectrum of neurological diseases. The aim of the present work was to elucidate the physiological role of SLC25A46 through the identification of its interactome with immunoprecipitation and proteomic analysis in whole cell extracts from the cerebellum, cerebrum, heart, and thymus of transgenic mice expressing ubiquitously SLC25A46-FLAG. Our analysis identified 371 novel putative interactors of SLC25A46 and confirmed 17 known ones. A total of 79 co-immunoprecipitated proteins were common in two or more tissues, mainly participating in mitochondrial activities such as oxidative phosphorylation (OXPHOS) and ATP production, active transport of ions or molecules, and the metabolism. Tissue-specific co-immunoprecipitated proteins were enriched for synapse annotated proteins in the cerebellum and cerebrum for metabolic processes in the heart and for nuclear processes and proteasome in the thymus. Our proteomic approach confirmed known mitochondrial interactors of SLC25A46 including MICOS complex subunits and also OPA1 and VDACs, while we identified novel interactors including the ADP/ATP translocases SLC25A4 and SLC25A5, subunits of the OXPHOS complexes and F₁F_o-ATP synthase, and components of the mitochondria-ER contact sites. Our results show that SLC25A46 interacts with a large number of proteins and protein complexes involved in the mitochondria architecture, energy production, and flux and also in inter-organellar contacts.

The Role of Mitochondria in Optic Atrophy With Autosomal Inheritance

Optic atrophy (OA) with autosomal inheritance is a form of optic neuropathy characterized by the progressive and irreversible loss of vision. In some cases, this is accompanied by additional, typically neurological, extra-ocular symptoms. Underlying the loss of vision is the specific degeneration of the retinal ganglion cells (RGCs) which form the optic nerve. Whilst autosomal OA is genetically heterogenous, all currently identified causative genes appear to be associated with mitochondrial organization and function. However, it is unclear why RGCs are particularly vulnerable to mitochondrial aberration. Despite the relatively high prevalence of this disorder, there are currently no approved treatments. Combined with the lack of knowledge concerning the mechanisms through which aberrant mitochondrial function leads to RGC death, there remains a clear need for further research to identify the underlying mechanisms and develop treatments for this condition. This review summarizes the genes known to be causative of autosomal OA and the mitochondrial dysfunction caused by pathogenic mutations. Furthermore, we discuss the suitability of available in vivo models for autosomal OA with regards to both treatment development and furthering the understanding of autosomal OA pathology.

Reanalysis of Exome Data Identifies Novel SLC25A46 Variants Associated with Leigh Syndrome

SLC25A46 (solute carrier family 25 member 46) mutations have been linked to various neurological diseases with recessive inheritance, including Leigh syndrome, optic atrophy, and lethal congenital pontocerebellar hypoplasia. SLC25A46 is expressed in the outer membrane of mitochondria, where it plays a critical role in mitochondrial dynamics. A deceased 7-month-old female infant was suspected to have Leigh syndrome. Clinical exome sequencing was non-diagnostic, but research reanalysis of the sequencing data identified two novel variants in SLC25A46: a missense (c.1039C>T, p.Arg347Cys; NM_138773, hg19) and a donor splice region variant (c.283+5G>A) in intron 1. Both variants were predicted to be damaging. Sanger sequencing of cDNA detected a single missense allele in the patient compared to control, and the SLC25A46 transcript levels were also reduced due to the splice region variant. Additionally, Western blot analysis of whole-cell lysate showed a decrease of SLC25A46 expression in proband fibroblasts, relative to control cells. Further, analysis of mitochondrial morphology revealed evidence of increased fragmentation of the mitochondrial network in proband fibroblasts, compared to control cells. Collectively, our findings suggest that these novel variants in SLC24A46, the donor splice one and the missense variant, are the cause of the neurological phenotype in this proband.

Players in Mitochondrial Dynamics and Female Reproduction

Mitochondrial dynamics (fission and fusion) are essential physiological processes for mitochondrial metabolic function, mitochondrial redistribution, and mitochondrial quality control. Various proteins are involved in regulating mitochondrial dynamics. Aberrant expression of these proteins interferes with mitochondrial dynamics and induces a range of diseases. Multiple therapeutic approaches have been developed to treat the related diseases in recent years, but their curative effects are limited. Meanwhile, the role of mitochondrial dynamics in female reproductive function has attracted progressively more attention, including oocyte development and maturation, fertilization, and embryonic development. Here, we reviewed the significance of mitochondrial dynamics, proteins involved in mitochondrial dynamics, and disorders resulting from primary mitochondrial dynamic dysfunction. We summarized the latest therapeutic approaches of hereditary mitochondrial fusion-fission abnormalities and reviewed the recent advances in female reproductive mitochondrial dynamics.

A Slc25a46 Mouse Model Simulating Age-Associated Motor Deficit, Redox Imbalance, and Mitochondria Dysfunction

The mitochondrial theory of aging postulates that accumulation of mtDNA mutations and mitochondrial dysfunction are responsible for producing aging phenotypes. To more comprehensively explore the complex relationship between aging and mitochondria dysfunction, we have developed a mouse model with Slc25a46 knockout, a nuclear gene described as encoding mitochondrial carriers, by CRISPR/Cas9 gene editing to mimic some typical aging phenotypes in human. Slc25a46-/- mice present segmental premature aging phenotypes characterized by shortened life span of no more than 2 months, obviously defective motor ability, gastrocnemius muscle atrophy, and imbalance of redox level in brain and liver. The underlying mechanism for multiple organ disorder may attribute to mitochondrial dysfunction, which is mainly manifested in the damaged mitochondrial structure (eg, vacuolar structure, irregular swelling, and disorganized cristae) and an age-associated decrease in respiratory chain enzyme (mainly complex I and IV) activity. In summary, our study suggests that the Slc25a46-/- mouse is a valid animal model for segmental aging-related pathologies studies based on mitochondrial theory, generating a new platform to both understand mechanisms between aging and mitochondria dysfunction as well as to design mitochondria-based therapeutic strategies to improve mitochondrial quality, and thereby the overall healthspan.

MICOS and the mitochondrial inner membrane morphology - when things get out of shape

Mitochondria play a key role in cellular signalling, metabolism and energetics. Proper architecture and remodelling of the inner mitochondrial membrane are essential for efficient respiration, apoptosis and quality control in the cell. Several protein complexes including mitochondrial contact site and cristae organizing system (MICOS), F₁ F_O -ATP synthase, and Optic Atrophy 1 (OPA1), facilitate formation, maintenance and stability of cristae membranes. MICOS, the F₁ F_O -ATP synthase, OPA1 and inner membrane phospholipids such as cardiolipin and phosphatidylethanolamine interact with each other to organize the inner membrane ultra-structure and remodel cristae in response to the cell's demands. Functional alterations in these proteins or in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine result in an aberrant inner membrane architecture and impair mitochondrial function. Mitochondrial dysfunction and abnormalities hallmark several human conditions and diseases including neurodegeneration, cardiomyopathies and diabetes mellitus. Yet, they have long been regarded as secondary pathological effects. This review discusses emerging evidence of a direct relationship between protein- and lipid-dependent regulation of the inner mitochondrial membrane morphology and diseases such as fatal encephalopathy, Leigh syndrome, Parkinson's disease, and cancer.

Systemic administration of AAV-Slc25a46 mitigates mitochondrial neuropathy in Slc25a46-/- mice

Mitochondrial disorders are the result of nuclear and mitochondrial DNA mutations that affect multiple organs, with the central and peripheral nervous system often affected. Currently, there is no cure for mitochondrial disorders. Currently, gene therapy offers a novel approach for treating monogenetic disorders, including nuclear genes associated with mitochondrial disorders. We utilized a mouse model carrying a knockout of the mitochondrial fusion-fission-related gene solute carrier family 25 member 46 (Slc25a46) and treated them with neurotrophic AAV-PHP.B vector carrying the mouse Slc25a46 coding sequence. Thereafter, we used immunofluorescence staining and western blot to test the transduction efficiency of this vector. Toluidine blue staining and electronic microscopy were utilized to assess the morphology of optic and sciatic nerves following treatment, and the morphology and respiratory chain activity of mitochondria within these tissues were determined as well. The adeno-associated virus (AAV) vector effectively transduced in the cerebrum, cerebellum, heart, liver and sciatic nerves. AAV-Slc25a46 treatment was able to rescue the premature death in the mutant mice (Slc25a46-/-). The treatment-improved electronic conductivity of the peripheral nerves increased mobility and restored mitochondrial complex activities. Most notably, mitochondrial morphology inside the tissues of both the central and peripheral nervous systems was normalized, and the neurodegeneration, chronic neuroinflammation and loss of Purkinje cell dendrites observed within the mutant mice were alleviated. Overall, our study shows that AAV-PHP.B's neurotrophic properties are plausible for treating conditions where the central nervous system is affected, such as many mitochondrial diseases, and that AAV-Slc25a46 could be a novel approach for treating SLC25A46-related mitochondrial disorders.

Nanoscopic quantification of sub-mitochondrial morphology, mitophagy and mitochondrial dynamics in living cells derived from patients with mitochondrial diseases

SLC25A46 mutations have been found to lead to mitochondrial hyper-fusion and reduced mitochondrial respiratory function, which results in optic atrophy, cerebellar atrophy, and other clinical symptoms of mitochondrial disease. However, it is generally believed that mitochondrial fusion is attributable to increased mitochondrial oxidative phosphorylation (OXPHOS), which is inconsistent with the decreased OXPHOS of highly-fused mitochondria observed in previous studies. In this paper, we have used the live-cell nanoscope to observe and quantify the structure of mitochondrial cristae, and the behavior of mitochondria and lysosomes in patient-derived SLC25A46 mutant fibroblasts. The results show that the cristae have been markedly damaged in the mutant fibroblasts, but there is no corresponding increase in mitophagy. This study suggests that severely damaged mitochondrial cristae might be the predominant cause of reduced OXPHOS in SLC25A46 mutant fibroblasts. This study demonstrates the utility of nanoscope-based imaging for realizing the sub-mitochondrial morphology, mitophagy and mitochondrial dynamics in living cells, which may be particularly valuable for the quick evaluation of pathogenesis of mitochondrial morphological abnormalities.

Genetic Neuropathy Due to Impairments in Mitochondrial Dynamics

Mitochondria are dynamic organelles capable of fusing, dividing, and moving about the cell. These properties are especially important in neurons, which in addition to high energy demand, have unique morphological properties with long axons. Notably, mitochondrial dysfunction causes a variety of neurological disorders including peripheral neuropathy, which is linked to impaired mitochondrial dynamics. Nonetheless, exactly why peripheral neurons are especially sensitive to impaired mitochondrial dynamics remains somewhat enigmatic. Although the prevailing view is that longer peripheral nerves are more sensitive to the loss of mitochondrial motility, this explanation is insufficient. Here, we review pathogenic variants in proteins mediating mitochondrial fusion, fission and transport that cause peripheral neuropathy. In addition to highlighting other dynamic processes that are impacted in peripheral neuropathies, we focus on impaired mitochondrial quality control as a potential unifying theme for why mitochondrial dysfunction and impairments in mitochondrial dynamics in particular cause peripheral neuropathy.

Mitochondrial genome variant m.3250T>C as a possible risk factor for mitochondrial cardiomyopathy

The MT-TL1 gene codes for the mitochondrial leucine transfer RNA (tRNA^Leu(UUR) ) necessary for mitochondrial translation. Pathogenic variants in the MT-TL1 gene result in mitochondriopathy in humans. The m.3250T>C variant in the MT-TL1 gene has been previously associated with exercise intolerance and mitochondrial myopathy, yet disease classification for this variant has not been consistently reported. Molecular studies suggest the m.3250T>C variant does not alter tRNA^Leu(UUR) structure but may have a modest impact on aminoacylation capacity. However, functional studies are limited. Our study aimed to further define the clinical presentation, inheritance pattern, and molecular pathology of the m.3250T>C variant. Families with the m.3250T>C variant were recruited from the Mitochondrial Disease Clinic at Cincinnati Children's Hospital Medical Center and GeneDx laboratory database. Affected individuals most frequently presented with cardiac findings, exercise intolerance, and muscle weakness. Hypertrophic cardiomyopathy was the most frequent cardiac finding. Many asymptomatic individuals had homoplasmic or near homoplasmic levels of the m.3250T>C variant, suggesting the penetrance is incomplete. Patient-derived fibroblasts demonstrated lowered ATP production and increased levels of reactive oxygen species. Our results demonstrate that the m.3250T>C variant exhibits incomplete penetrance and may be a possible cause of cardiomyopathy by impacting cellular respiration in mitochondria.

Recent Advances in Drosophila Models of Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited peripheral neuropathies. CMT patients typically show slowly progressive muscle weakness and sensory loss in a distal dominant pattern in childhood. The diagnosis of CMT is based on clinical symptoms, electrophysiological examinations, and genetic testing. Advances in genetic testing technology have revealed the genetic heterogeneity of CMT; more than 100 genes containing the disease causative mutations have been identified. Because a single genetic alteration in CMT leads to progressive neurodegeneration, studies of CMT patients and their respective models revealed the genotype-phenotype relationships of targeted genes. Conventionally, rodents and cell lines have often been used to study the pathogenesis of CMT. Recently, Drosophila has also attracted attention as a CMT model. In this review, we outline the clinical characteristics of CMT, describe the advantages and disadvantages of using Drosophila in CMT studies, and introduce recent advances in CMT research that successfully applied the use of Drosophila, in areas such as molecules associated with mitochondria, endosomes/lysosomes, transfer RNA, axonal transport, and glucose metabolism.

Systemic Delivery of AAV-Fdxr Mitigates the Phenotypes of Mitochondrial Disorders in Fdxr Mutant Mice

Gene therapy now provides a novel approach for treating inherited monogenetic disorders, including nuclear gene mutations associated with mitochondrial diseases. In this study, we have utilized a mouse model carrying a p.Arg389Gln mutation of the mitochondrial Ferredoxin Reductase gene (Fdxr) and treated them with neurotropic AAV-PHP.B vector loaded with the mouse Fdxr cDNA sequence. We then used immunofluorescence staining and western blot to test the transduction efficiency of this vector. Toluidine blue staining and electronic microscopy were also utilized to assess the morphology of optic and sciatic nerves, and the mitochondrial respiratory chain activity was determined as well. The AAV vector effectively transduced in the central nervous system and peripheral organs, and AAV-Fdxr treatment reversed almost all the symptoms of the mutants (Fdxr^R389Q/R389Q). This therapy also improved the electronic conductivity of the sciatic nerves, prevented optic atrophy, improved mobility, and restored mitochondrial complex function. Most notably, the sensory neuropathy, neurodegeneration, and chronic neuroinflammation in the brain were alleviated. Overall, we present the first demonstration of a potential definitive treatment that significantly improves optic and sciatic nerve atrophy, sensory neuropathy, and mitochondrial dysfunction in FDXR-related mitochondriopathy. Our study provides substantial support for the translation of AAV-based Fdxr gene therapy into clinical applications.

Integrated analysis of the molecular pathogenesis of FDXR-associated disease

The mitochondrial flavoprotein ferredoxin reductase (FDXR) is required for biogenesis of iron-sulfur clusters and for steroidogenesis. Iron-sulfur (Fe-S) clusters are ubiquitous cofactors essential to various cellular processes, and an increasing number of disorders are associated with disruptions in the synthesis of Fe-S clusters. Our previous studies have demonstrated that hypomorphic mutations in FDXR cause a novel mitochondriopathy and optic atrophy in humans and mice, attributed in part to reduced function of the electron transport chain (ETC) as well as elevated production of reactive oxygen species (ROS). Inflammation and peripheral neuropathy are also hallmarks of this disease. In this paper, we demonstrate that FDXR mutation leads to significant optic transport defects that are likely to underlie optic atrophy, a major clinical presentation in FDXR patients, as well as a neurodegenerative loss of cells in the central nervous system (CNS). Molecular analysis indicates that FDXR mutation also leads to mitochondrial iron overload and an associated depolarization of the mitochondrial membrane, further supporting the hypothesis that FDXR mutations cause neurodegeneration by affecting FDXR's critical role in iron homeostasis.

Neuron-specific knockdown of solute carrier protein SLC25A46a induces locomotive defects, an abnormal neuron terminal morphology, learning disability, and shortened lifespan

Various mutations in the SLC25A46 gene have been reported in mitochondrial diseases that are sometimes classified as type 2 Charcot-Marie-Tooth disease, optic atrophy, and Leigh syndrome. Although human SLC25A46 is a well-known transporter that acts through the mitochondrial outer membrane, the relationship between neurodegeneration in these diseases and the loss-of-function of SLC25A46 remains unclear. Two Drosophila genes, CG8931 (dSLC25A46a) and CG5755 (dSLC25A46b) have been identified as candidate homologs of human SLC25A46. We previously characterized the phenotypes of pan-neuron-specific dSLC25A46b knockdown flies. In the present study, we developed pan-neuron-specific dSLC25A46a knockdown flies and examined their phenotypes. Neuron-specific dSLC25A46a knockdown resulted in reduced mobility in larvae as well as adults. An aberrant morphology for neuromuscular junctions (NMJs), such as a reduced synaptic branch length and decreased number and size of boutons, was observed in dSLC25A46a knockdown flies. Learning ability was also reduced in the larvae of knockdown flies. In dSLC25A46a knockdown flies, mitochondrial hyperfusion was detected in NMJ synapses together with the accumulation of reactive oxygen species and reductions in ATP. These phenotypes were very similar to those of dSLC25A46b knockdown flies, suggesting that dSLC25A46a and dSLC25A46b do not have redundant roles in neurons. Collectively, these results show that the depletion of SLC25A46a leads to mitochondrial defects followed by an aberrant synaptic morphology, resulting in locomotive defects and learning disability. Thus, the dSLC25A46a knockdown fly summarizes most of the phenotypes in patients with mitochondrial diseases, offering a useful tool for studying these diseases.

The role of mitochondrial-related nuclear genes in age-related common disease

Mitochondria are critical organelles that provide energy as ATP to the cell. Besides 37 genes encoded by mitochondrial genome, it has been estimated that over 1500 nuclear genes are required for mitochondrial structure and function. Thus, mutations of many genes in the nuclear genome cause dysfunction of mitochondria that can lead to many severe conditions. Mitochondrial dysfunction often results in reduced ATP synthesis, higher levels of reactive oxygen species (ROS), imbalanced mitochondrial dynamics, and other detrimental effects. In addition to rare primary mitochondrial disorders, these mitochondrial-related genes are often associated with many common diseases. For example, in neurodegenerative diseases such as Parkinson's, Alzheimer's, and Huntington disease, mitochondrialand energy metabolism abnormalities can greatly affect brain function. Cancer cells are also known to exhibit repressed mitochondrial ATP production in favor of glycolysis, which fuels the aggressive proliferation and metastasis of tumor tissues, leading many to speculate on a possible relationship between compromised mitochondrial function and cancer. The association between mitochondrial dysfunction and diabetes is also unsurprising, given the organelle's crucial role in cellular energy utilization. Here, we will discuss the multiple lines of evidence connecting mitochondrial dysfunction associated with mitochondria-related nuclear genes to many of the well-known disease genes that also underlie common disease.

20,000 picometers under the OMM: diving into the vastness of mitochondrial metabolite transport

The metabolic compartmentalization enabled by mitochondria is key feature of many cellular processes such as energy conversion to ATP production, redox balance, and the biosynthesis of heme, urea, nucleotides, lipids, and others. For a majority of these functions, metabolites need to be transported across the impermeable inner mitochondrial membrane by dedicated carrier proteins. Here, we examine the substrates, structural features, and human health implications of four mitochondrial metabolite carrier families: the SLC25A family, the mitochondrial ABCB transporters, the mitochondrial pyruvate carrier (MPC), and the sideroflexin proteins.

Genetic compensation in a stable slc25a46 mutant zebrafish: A case for using F0 CRISPR mutagenesis to study phenotypes caused by inherited disease

A phenomenon of genetic compensation is commonly observed when an organism with a disease-bearing mutation shows incomplete penetrance of the disease phenotype. Such incomplete phenotypic penetrance, or genetic compensation, is more commonly found in stable knockout models, rather than transient knockdown models. As such, these incidents present a challenge for the disease modeling field, although a deeper understanding of genetic compensation may also hold the key for novel therapeutic interventions. In our study we created a knockout model of slc25a46 gene, which is a recently discovered important player in mitochondrial dynamics, and deleterious mutations in which are known to cause peripheral neuropathy, optic atrophy and cerebellar ataxia. We report a case of genetic compensation in a stable slc25a46 homozygous zebrafish mutant (hereafter referred as “mutant”), in contrast to a penetrant disease phenotype in the first generation (F0) slc25a46 mosaic mutant (hereafter referred as “crispant”), generated with CRISPR/Cas-9 technology. We show that the crispant phenotype is specific and rescuable. By performing mRNA sequencing, we define significant changes in slc25a46 mutant’s gene expression profile, which are largely absent in crispants. We find that among the most significantly altered mRNAs, anxa6 gene stands out as a functionally relevant player in mitochondrial dynamics. We also find that our genetic compensation case does not arise from mechanisms driven by mutant mRNA decay. Our study contributes to the growing evidence of the genetic compensation phenomenon and presents novel insights about Slc25a46 function. Furthermore, our study provides the evidence for the efficiency of F0 CRISPR screens for disease candidate genes, which may be used to advance the field of functional genetics.

The ‘mitochondrial contact site and cristae organising system’ (MICOS) in health and human disease

The ‘mitochondrial contact site and cristae organising system’ (MICOS) is an essential protein complex that promotes the formation, maintenance and stability of mitochondrial cristae. As such, loss of core MICOS components disrupts cristae structure and impairs mitochondrial function. Aberrant mitochondrial cristae morphology and diminished mitochondrial function is a pathological hallmark observed across many human diseases such as neurodegenerative conditions, obesity and diabetes mellitus, cardiomyopathy, and in muscular dystrophies and myopathies. While mitochondrial abnormalities are often an associated secondary effect to the pathological disease process, a direct role for the MICOS in health and human disease is emerging. This review describes the role of MICOS in the maintenance of mitochondrial architecture and summarizes both the direct and associated roles of the MICOS in human disease.

Biallelic mutations in FDXR cause neurodegeneration associated with inflammation

Mitochondrial dysfunction lies behind many neurodegenerative disorders, owing largely to the intense energy requirements of most neurons. Such mitochondrial dysfunction may work through a variety of mechanisms, from direct disruption of the electron transport chain to abnormal mitochondrial biogenesis. Recently, we have identified biallelic mutations in the mitochondrial flavoprotein "ferredoxin reductase" (FDXR) gene as a novel cause of mitochondriopathy, peripheral neuropathy, and optic atrophy. In this report, we expand upon those results by describing two new cases of disease-causing FDXR variants in patients with variable severity of phenotypes, including evidence of an inflammatory response in brain autopsy. To investigate the underlying pathogenesis, we examined neurodegeneration in a mouse model. We found that Fdxr mutant mouse brain tissues share pathological changes similar to those seen in patient autopsy material, including increased astrocytes. Furthermore, we show that these abnormalities are associated with increased levels of markers for both neurodegeneration and gliosis, with the latter implying inflammation as a major factor in the pathology of Fdxr mutations. These data provide further insight into the pathogenic mechanism of FDXR-mediated central neuropathy, and suggest an avenue for mechanistic studies that will ultimately inform treatment.

Insights into the genotype-phenotype correlation and molecular function of SLC25A46

Recessive SLC25A46 mutations cause a spectrum of neurodegenerative disorders with optic atrophy as a core feature. We report a patient with optic atrophy, peripheral neuropathy, ataxia, but not cerebellar atrophy, who is on the mildest end of the phenotypic spectrum. By studying seven different nontruncating mutations, we found that the stability of the SLC25A46 protein inversely correlates with the severity of the disease and the patient's variant does not markedly destabilize the protein. SLC25A46 belongs to the mitochondrial transporter family, but it is not known to have transport function. Apart from this possible function, SLC25A46 forms molecular complexes with proteins involved in mitochondrial dynamics and cristae remodeling. We demonstrate that the patient's mutation directly affects the SLC25A46 interaction with MIC60. Furthermore, we mapped all of the reported substitutions in the protein onto a 3D model and found that half of them fall outside of the signature carrier motifs associated with transport function. We thus suggest that there are two distinct molecular mechanisms in SLC25A46-associated pathogenesis, one that destabilizes the protein while the other alters the molecular interactions of the protein. These results have the potential to inform clinical prognosis of such patients and indicate a pathway to drug target development.

Novel Drosophila model for mitochondrial diseases by targeting of a solute carrier protein SLC25A46

Mutations in SLC25A46 gene have been identified in mitochondrial diseases that are sometimes classified as Charcot-Marie-Tooth disease type 2, optic atrophy and Leigh syndrome. Human SLC25A46 functions as a transporter across the outer mitochondrial membrane. However, it is still unknown how the neurodegeneration occurring in these diseases relates to the loss of SLC25A46 function. Drosophila has CG5755 (dSLC25A46) as a single human SLC25A46 homolog. Here we established pan-neuron specific dSLC25A46 knockdown flies, and examined their phenotypes. Neuron specific knockdown of dSLC25A46 resulted in an impaired motility in both larvae and adults. Defects at neuromuscular junctions (NMJs), such as reduced synaptic branch length, decreased number and size of bouton, reduced density and size of active zone were also observed with the dSLC25A46 knockdown flies. Mitochondrial hyperfusion in synapse at NMJ, accumulation of reactive oxygen species and reduction of ATP were also observed in the dSLC25A46 knockdown flies. These results indicate that depletion of SLC25A46 induces mitochondrial defects accompanied with aberrant morphology of motoneuron and reduction of active zone that results in defect in locomotive ability. In addition, it is known that SLC25A46 mutations in human cause optic atrophy and knockdown of dSLC25A46 induces aberrant morphology of optic stalk of photoreceptor neurons in third instar larvae. Morphology and development of optic stalk of photoreceptor neurons in Drosophila are precisely regulated via cell proliferation and migration. Immunocytochemical analyses of subcellular localization of dSLC25A46 revealed that dSLC25A46 localizes not only in mitochondria, but also in plasma membrane. These observations suggest that in addition to the role in mitochondrial function, plasma membrane-localized dSLC25A46 plays a role in cell proliferation and/or migration to control optic stalk formation. The dSLC25A46 knockdown fly thus recapitulates most of the phenotypes in mitochondrial disease patients, providing a useful tool to study these diseases.

Mitochondrial Dynamics in Basal and Stressful Conditions

The historical role of mitochondria resides in converting the energy released during the oxidation of macromolecules (carbohydrates, lipids and proteins) into adenosine tri-phosphate, a major form of chemically stored energy which sustains cell growth and homeostasis. Beyond this role in bioenergetics regulation, mitochondria play a role in several other cellular processes including lipid metabolism, cellular calcium homeostasis, autophagy and immune responses. Furthermore, mitochondria are highly dynamic organelles: as all other cellular endomembranes, they are continuously moving along cytoskeleton, and, most importantly, they constantly interact one with each other by membrane tethering, fusion and fission. This review aims to highlight the tight correlation between the morphodynamics of mitochondria and their biological function(s), in physiological as well as stress conditions, in particular nutrient deprivation, pathogen attack and some human diseases. Finally, we emphasize some crosstalk between the fusion/fission machinery and the autophagy pathway to ending on some speculative hypothesis to inspire future research in the field.